Light-emitting diodes (LEDs) based on solid state lighting technology are likely to replace incandescent and fluorescent bulbs in housing and commercial markets. The modules, bulbs, and fixtures made out of the LEDs/solid state lighting have distinct advantages such as, they are brighter, require less energy, and extend the light's lifetime. Some of the electricity in an LED becomes heat rather than light and if the heat is not removed, the LEDs run at high temperatures, which not only lowers their efficiency, but also makes the LED less reliable and cost effective alternate to current lighting industry. In order to solve the difficult problem in this field, the LED industry has developed many different methods and solutions in addressing key problems such as heat dissipation, shape, luminance, product life and cost. The high power LED illumination still faces a lot of technological challenges at present with regards to improving its efficiency in heat transfer.
The current solution in thermal management involves materials acting as a heat sink made from aluminum, although copper may be used as well. New classes of materials including thermoplastic compounds that are used when heat dissipation requirements are lower than normal or have a complex shape would offer improvements by molding and extrusion which would offer better thermal transfer than copper with a lower weight than aluminum. The improved thermoplastic composition further offers the advantage of being able to be formed into complex two-dimensional parts and can also be easily mass produced.
The present invention relates to a heat dissipating thermoplastic compound composition acting as a both external and internal heat sink which can be easily customized for any given shape and size via regular melt processing techniques in a LED part design. Such a composition can solve the problems of light decay, heat dissipation, and shape adoption in next generation high brightness LEDs.
Another aspect of the current invention is that the thermally conducting composition can be formulated to provide a superior surface finish which provides superior thermal radiation of heat especially at higher temperatures. This is particularly useful in cases where heat is dissipated by radiation and the thermally conductive composition is acting as a reflector. Moreover, a perfectly fused contact area or good interface allows the use of a thinner layer of thermal compound, which will reduce the thermal resistance between the heat sink and LED source.
The prior art in thermally conductive thermoplastic compounds uses boron nitride as a conductive filler. Thermally conductive fillers do not only have to be limited to boron nitride. The prior art also teaches that there are a variety of other fillers available for compounding with thermoplastics that will impart thermal conductivity to a polymer compound. These materials include but are not limited to metals and their alloys, such as copper, aluminum, bronze, gold, silver, iron, lead, stainless steel, titanium, brass, nickel coated fibers, aluminum coated fibers, and metal oxides, such as zinc oxide, aluminum oxide, beryllium oxide, magnesium oxide, iron oxide, and aluminum nitride. The use of ceramics and minerals as thermally conductive fillers is also described in the prior art. These thermally conductive materials include granite, silicon carbide, zirconium silicate, limestone, marble, quartz, and sandstone. Thermally conductive fillers that are known in the prior art also include carbon rich materials such as carbon fibers, carbon nanotubes, carbon nanofibers, diamond, natural and synthetic unexpanded graphite, natural and synthetic expanded graphite, carbon black, diamond powders (synthetic and natural), and graphene.
A limitation of some of the fillers known in the prior art, including boron nitrides, is that thermal conductivity is only realized uni-directionally and therefore through-plane thermal conductivity is delivered at a level which is less than that needed in some important commercial applications. However, prior art teaches that the thermal conductivity of thermoplastic compositions can be increased by the addition of another filler having a low aspect ratio. There is accordingly a need for a composition which can increase through plane thermal conductivity over the levels provided by boron nitride and other conventional thermally conductive fillers.
In addition to having thermal conductivity in some applications it is also critical for thermoplastic compound formulations to impart additional performance characteristics including electrical conductivity, electrical insulativity, processability and flame retardancy. In many cases, enhanced electrical conductivity facilitates electroplating, electroless plating, and primer free electrostatic painting of thermally conductive polymer parts. These plating/painting processes are typically performed by moving ions in an electrolytic solution by an electric field over a given conductive substrate in order to coat said substrate with a thin layer of the material, typically a metal. Thermoplastic material that can be electroplated yields also generally benefits by providing improved wear and mar resistance, lubricity, aesthetics, and corrosion resistance.
The improvement of electrical insulation characteristics also normally contributes to a higher break down voltage of thermally conductive polymer parts. This property is very desirable in numerous electrical and electronic applications. For example in LEDs, an electrically insulative and thermally conductive polymer compound can be used in place of electrically conductive aluminum for making housings, back plates, connectors and sleeves with better results. In these parts, a higher breakdown voltage is typically desired.
Flame retardant chemicals, compounds, and fillers can also be introduced into thermoplastic compounds to improve their flammability ratings. Introducing flame retardants into the thermally conductive thermoplastic compounds helps impart ignition resistance and inhibits or resists the spread of fire. This added property allows for the thermally conductive thermoplastic compound to be utilized safely where a potential for fire exists, meeting the requirements of Underwriter Labs UL-94 and the International Electrotechnical Commission (IEC) IEC 60707, IEC 60695-11-10, and IEC 60695-11-20.
Processability is a very important criterion in manufacturing most articles via conventional molding and extrusion techniques. Good processability is a beneficial characteristic for thermoplastics that are used in almost all applications ranging from conventional injection molding to complex techniques used in specialty applications. However, excellent processability can be critical in many complex molding applications that require a very fine balance of performance properties along with low temperature melting point and easy flow.